There are certain methods currently available for measuring internal stress in a concrete structure, for example, a method of measuring current stress by providing in advance a marker or a sensor (for example, see Patent Literature 1) or a method of sampling a core and making an estimation from the amount of rebound or the AE Kayser effect (for example, see Patent Literature 2).
Then, for example, suppose that a concrete structure is subjected momentarily to an intensive force due to a natural disaster such as an earthquake or a tornado or an accident such as a collision with a high-speed traffic vehicle. Even in this case, the fact is that when the concrete structure itself may not be destroyed but only elastically deformed, and then restored from the elastic deformation by the external force being subsequently released, the magnitude and the distribution of the force to which the concrete structure has been subjected cannot be accurately estimated.
In such a case, it is conceivable to analyse the stress history of the concrete structure, for example, by making an estimation by simulation or by sampling a core at an expectedly significant point so as to estimate the maximum amount of stress by the acoustic emission (AE measurement) of the core. However, any one of the methods cannot serve to provide an evaluation with high reliability. That is, the estimation method by simulation has a problem that the result of a simulation remains still ambiguous because the magnitude and the direction of a suddenly acting external force are only a matter of speculation in the first place.
Furthermore, the AE measurement cannot be made with high accuracy because the AE measurement is conducted by loading a core material onto a mechanical tester and then estimating a preceding maximum stress on the basis of the point of increase of a microfracture sound. Furthermore, the AE measurement has still another problem, for example, that the AE measurement cannot serve as a reproducible test because the direction of the maximum, stress at the time of occurrence of an event cannot be identified, and only a limited number of inspections can be conducted on the entire structure because the AE measurement is a destructive testing.
Still furthermore, also conceivable is such a method of placing a number of strain gauges in an entire concrete structure and then analysing the stress history of the concrete structure by monitoring the same all the time. However, such a method is not practical for a typical building, and embedding a sensor, which is a foreign matter, in the concrete may also lead to non-uniformity, thereby causing the concrete to be destroyed.
As means to address the problems mentioned above, the present inventors have proposed a method of mixing, particle by particle, synthetic calcites serving as a stress indicator into an object to be measured; and repeatedly measuring a number of calcite particles one by one with a microscope for the twinning density of the calcite particles, thereby estimating the stress that has acted upon the entire object to be measured on the basis of the average value of the resulting twinning density (see Patent Literature 3).
Such a measurement method is realistic because the method employs the property of calcite crystals, so that the need for maintenance is eliminated and measurements may be conducted only when required.